(1) Field of the Invention
The present invention relates to a private branch exchange system having connected thereto an attendant console and a plurality of extensions.
In private branch exchange systems, an attendant console is connected thereto for responding to an incoming call from a telephone line which is connected with a public telephone network, and calling a party on an extension to which a calling party requests to speak. Conventionally, in middle or large-size private branch exchange systems, a plurality of attendant consoles are provided in an attendant console room which is located near the main frame of the private branch exchange system. However, recently, in many factories and offices, the plurality of attendant consoles are located dispersively, for example, in reception offices and secretaries' offices in a plurality of divisions or departments. Generally, a large number of control signals are required to be transmitted between the main frame of the private branch exchange system and the attendant console through long distances.
(2) Description of the Related Art
FIG. 1 shows a construction of a conventional private branch exchange system. In FIG. 1, reference numeral 30 denotes a public telephone line, 40 denotes the main frame of the private branch exchange system, 41 denotes a trunk circuit connecting with the public telephone line, 42 denotes a switch circuit, 431 and 432 each denote an attendant console control circuit, 44 denotes a central control circuit, 441 and 442 each denote an attendant console, 461 and 462 each denote a set of transmission lines connected between an attendant console and the main frame of the private branch exchange system, 451 and 453 each denote a lamp indicating portion, 452 and 454 each denote a key input portion, and 50 denotes an extension.
In the construction of FIG. 1, when a call is incoming in the trunk circuit 41, the central control circuit 44 sends a control signal which indicates that a call is incoming to all the attendant consoles 441 and 442 through the corresponding attendant console control circuits 431 and 432. Responding to the signal, lamp indicators in the lamp indicating portions 451 and 453 which indicate an incoming call from the public telephone line 30, are turned on in each of the attendant consoles 441 and 442. When an operator at one of the attendant consoles 441 and 442 pushes a key the input from which generates a control signal which indicates that the operator at the attendant console will respond to the incoming call, the control signal from the attendant console is transmitted to the central control circuit 44 through the corresponding attendant console control circuit. Responding to the control signal which is first received in the central control circuit 44, the central control circuit 44 controls the switch circuit 42 to connect the above attendant console control circuit 431 from which the above control signal has been first sent to the central control circuit 44, to the trunk circuit 41. Thus, the operator at the attendant console 441 can speak with the calling party through the attendant console control circuit 431 and the trunk circuit 41, and can learn the party to which the calling party wishes to speak, by speaking with the calling party. When the operator at the attendant console 431 recognizes the party to which the calling party wishes to speak, the operator inputs the number of the extension of the party to which the calling party wishes to speak, from the key input portion 452. The number of the extension is transmitted from the attendant console 441 to the attendant console control circuit 431, and is transferred to the central control circuit 44. Receiving the number of the extension of the party to which the calling party wishes to speak, the central control circuit 44 controls the switch circuit 42 to connect the extension of the party to which the calling party wishes to speak, to the trunk circuit 41.
In the above operation, the man-machine interface at the attendant consoles 441 and 442 is realized by the keys in the key input portions 452 and 454 and the lamp indicators in the lamp indicating portions 451 and 453. The lamp indicators turn on and off responding to pushing of the keys, and it is required that the delay of the turn-on or turn-off of the lamp indicators in the response to the pushing of the keys is less than 200 msec so as to be natural to the operator.
In the prior art, the CCITT V.11 system is utilized for transmission of control signals between attendant console control circuits and corresponding attendant consoles. According to the CCITT V.11 recommendation, a balanced cable is used for transmission of control signals in each direction from an attendant console control circuit to a corresponding attendant console, and from the attendant console to the corresponding attendant console control circuit, i.e., a pair of balanced cables are laid between each of the attendant consoles 441 and 442 and the main frame 40 of the private branch exchange system, in addition to transmission lines for transmitting speech signals between each of the attendant consoles 441 and 442 and the main frame 40 of the private branch exchange system. As mentioned before, recently, the attendant consoles are located dispersively around the main frame of the private branch exchange system, and therefore, the distances between the attendant consoles and the main frame 40 of the private branch exchange system are large. However, generally, the above V11 system requires additional equipment for a long distance transmission, and the provision of the additional equipment increases cost for construction. Further, when the distances between the attendant consoles and the main frame 40 of the private branch exchange system are large, the cost in constructing the whole private branch exchange system is increased due to the above laying of the pair of balanced cables.